i don't have an answer for this but i had already given it some thought.
one thing that came to mind was that, as is, its a pretty inefficient market.
as other posters have noted, they have received passes, but never gotten around to using them. further, the people that get passes along w/ credit card are statistically probably pretty spread out around the country.

if you start allowing a secondary market for the passes, i could envision 2 undesirable outcomes:
1. the inefficient distribution of passes across the country suddenly becomes more efficient as there is an economic incentive for people who may not have used their passes to cash in...suddenly passes that would have maybe been used at a sleepy club like PDX if at all find themselves sold to people likely clustered around busier airports....busier airports, crowded clubs, and now increased supply of access to those already crowded clubs

2. potential for fraud...which a number people are suggesting here and what the OP even said the lounge agent explained that they were on the look out for